In the manufacture of certain wet-laid paper products such as facial tissue, bathroom tissue, or paper towels, the paper web is conventionally subjected to a creping process in order to give it desirable textural characteristics, such as softness and bulk. The creping process typically involves adhering the web to a rotating creping cylinder, such as the apparatus known as a Yankee dryer, and then dislodging the adhered web with a doctor blade. The impact of the web against the doctor blade ruptures some of the fiber-to-fiber bonds within the web and causes the web to wrinkle or pucker.
The severity of this creping action is dependent upon a number of factors, including the degree of adhesion between the web and the surface of the creping cylinder. Greater adhesion causes increased softness, although generally with some loss of strength. In order to increase adhesion, an adhesive creping aid is used to enhance any naturally occurring adhesion that the web may have due to its water content, which will vary widely depending on the extent to which the web has been previously dried. Creping aids should also prevent wear of the dryer surface and provide lubrication between the doctor blade and the dryer surface and reduce chemical corrosion, as well as controlling the extent of creping. A coating that adheres the sheet to the drum will give a good crepe, imparting absorbance and softness with the least possible loss of paper strength. If adhesion to the dryer drum is too strong, the sheet may pick or even “plug”, i.e., underride the doctor blade, and wrap around the dryer drum. If there is not enough adhesion, the sheet will lift off too easily and undergo too little creping. The creping adhesive, as an aqueous solution or dispersion, is usually sprayed onto the surface of the creping cylinder, e.g., a Yankee dryer. This adhesion of the sheet to the Yankee dryer improves heat transfer, allowing more efficient drying of the sheet.
If the web sticks too strongly to the creping cylinder, release agents can be sprayed on the cylinder. These release agents aid in the release of the tissue web at the creping blade, lubricate and protect the blade from excessive wear, and modify the properties of the adhesive allowing for control of coating thickness. Release agents may include materials such as emulsifiable oils, polyphosphates, and various surfactant-type chemistries. Release agents may be added to the wet end, blended with the adhesive and sprayed on the dryer using a single application system, or sprayed separately from the adhesive via a dedicated application system. The mode of action of release agents is such that they intentionally interfere with the formation of the coating. Typically, as the amount of release agent added is increased, the level of adhesion continues to decrease.
Certain hydrophobic chemicals, typically in the form of aqueous emulsions, are applied to paper machine equipment to act as release agents. For example, U.S. Pat. No. 5,658,374 to Glover teaches that an oil-in-water emulsion containing an alcohol, a fatty acid or an oil, and lecithin emulsified with a water-soluble or water-dispersible surfactant can be used to control sticky deposition on the surfaces of press rolls, yankee rolls and couch rolls surfaces in papermaking In another example, U.S. Pat. No. 5,863,385 to Siebott et. al. teaches a process for cleaning and preventing deposition on paper machine parts, including the press section, by treating the surface with an oil-in-water emulsion. The oil phase can be any of several compounds including saturated hydrocarbons, fatty alcohols, fatty acids, fatty acid esters, paraffin oil, mineral oil or poly-alpha-olefins. In another example, U.S. Pat. No. 6,139,911 to Vanhecke et. al. teaches the use of aqueous microemulsions for improving the release properties of press rolls where the oil phase is selected from oils, water insoluble surfactants, water insoluble polymers, and waxes. The microemulsion is applied by first diluting it with excess water or by applying it directly in the presence of excess water. When the microemulsion is applied to the press roll in either diluted manner, the emulsion breaks up, causing the release components to deposit on the roll surface as larger macroemulsion size (or greater) particles, which are more efficient at affecting release.
U.S. Pat. No. 6,558,513 to Pease et. al. teaches the use of non-curing hydrocarbon polymers, such as polybutene, for improving the release of paper webs from the surface of press rolls or other papermaking equipment or converting equipment. The composition is applied directly to the equipment surface in the absence of water, which would require application to an already cured creping adhesive coating and preclude its use in systems where the adhesive and release agents are blended and sprayed onto the dryer in a single application system. Further, these compositions contain, at minimum, 20% polybutene.